Plastic films that are superior in transparency and heat sealability in wide applications are required in general packaging applications, of which typical examples are bags for storing food, and in the fields of fishery, agriculture, building, medical, etc.
Transparency is normally indicated in terms of light beam transmittance, and the higher the transmittance, the more excellent the transparency. Films superior in transparency are used preferentially as packaging material because the contents can be seen from outside.
Heat sealing refers to a method in which films are superposed and joined together by heat and pressure using heating bars, heating plates, heating rolls, or the like.
Many of conventional plastic products, particularly plastic packaging materials are discarded soon after use in many cases, so that their disposal problems are pointed out. As representative general packaging plastics, polyethylene, polypropylene, polyethylene terephthalate (PET), etc. can be cited. These materials are high in calorific value produced during burning. Thus there is a fear that they may damage an incinerator during burning treatment. Further, polyvinyl chloride, which is large in the usage even now, hardly burns due to its self-extinguishing property. Also, in many cases, plastic products including such unburnable materials are buried. Due to their chemical and biological stability, they scarcely decompose and remain, so that they are causing problems such as short life of burial sites. Thus, ones that are low in calorie, decompose in the soil, and are safe are desired and many researches are being made.
As one example, there is a polylatic acid. It has a burning calory less than half that of polyethylene, and it turns to a harmless decomposed product in soil or water by the progression of hydrolysis and then by microorganisms. Currently, researches for obtaining articles formed of polylactic acids, specifically, films, sheets and bottles are being made.
But, a polylatic acid has an elongation of only 38% when pulled, and it is already known that it is a very brittle material. Thus, if it is made into a film, it is practically difficult to use without orienting. Thus, as disclosed in Japanese patent publication 9-111107, trials are being made to improve shock resistance by blending other aliphatic polyester by several parts by weight. But if such films are let to stand at a temperature slightly higher than room temperature, there was a problem that physical properties such as elongation at break and heat sealing strength change with time.
Patent publication 10-146936 proposes a bag of a biodegradable film using a laminated film which has an inner-layer film comprising a polylactic acid-family polymer and a specific aliphatic polyester, and an outer layer which is an oriented film of a polylactic acid-family polymer, and which is superior in heat sealability and transparency. But such a laminated film had a problem that it cannot be heat-sealed without applying high temperature, so that corrugation tends to develop in the outer-layer film by heat sealing.
Therefore, it is an object of this invention to provide a bag which can be heat sealed at low temperature, which does not develop corrugation and which has transparency and has degradability in natural environment.